This invention relates to an image reading device for reading image by scanning a document, an image forming apparatus such as a digital copying machine which forms an image by using the image reading apparatus, and an image forming method.
Conventionally, the change of the image magnification rate in the sub-scanning direction in a digital copying machine (DPPC) is performed by changing a scanning speed of a scanner for scanning a document in accordance with the magnification rate. The scanning speed of the scanner is determined by variably controlling the rotation number of a motor for driving the scanner. Accordingly, in changing the magnification rate in the sub-scanning direction, the number of effective data in the main scanning direction is determined by variably controlling the rotation number of the motor within the available magnification range defined in the specification of the machine.
According to the conventional method as mentioned above, the scanner must be controlled to change the scanning speed every time when the magnification rate is desired to be changed in order to obtain the set magnification rate. The scanner is controlled by changing the rotation number of the motor for driving the scanner in accordance with the desired magnification rate.
In a middle or low speed DPPC machine which is not required so high speed scanning operation (i.e., in a machine the scanning speed of the scanner of which is relatively low when the scanning speed is set at the highest level at which the image reduction rate is largest) within a variable range of the rotation number of the motor (i.e., within a variable range of the scanning speed of the scanner), the scanning speed of the scanner is reasonable in mechanical point of view with respect to the driving specification of the motor.
However, in a high-speed machine, the operation speed of the scanner is so high that some functional problems will occur. For example, due to the limitation generated by the acceleration condition of a stepping motor which drives a scanner, a necessary run-up distance of the scanner cannot be secured in the scanning width in the sub-scanning direction. In addition, the vibration of a carriage will be generated by the high-speed scanning operation, thereby the image obtained by the scanning will be distorted. According to the conventional method, the above-mentioned problems will be made so critical.